Lubricating oil composition

ABSTRACT

A lubricating oil composition comprising a zinc dithiophosphate and a zinc saturated aliphatic monocarboxylate dissolved in a base oil, wherein the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are in the ratio of equivalents from 6:1 to 45:1.

[0001] The present invention relates to a lubricating oil composition,and particularly relates to a lubricating oil composition favorablyemployable for hydraulic systems and power transmission devices.

BACKGROUND OF THE INVENTION

[0002] Lubricating oils are generally prepared by adding variousadditives to a base oil. Examples of base oils include mineral oil,synthetic oil, and natural oil, such as vegetable oil and its modifiedproducts.

[0003] Zinc dithiophosphate, such as zinc dialkyldithiophosphate or zincdialkylaryldithiophosphate, is employed in hydraulic oil for powertransmission systems of automobiles and hydraulic systems as a requisiteadditive because it imparts anti-oxidation property and anti-abrasionproperty to lubricating oils.

[0004] Recently, most of lubricating oils, such as hydraulic oils, areused under increased severe conditions, for instance, at elevatedtemperatures and for a prolonged time. The zinc dithiophosphate isstable under normal conditions but is liable to decompose at hightemperatures or after the use for a long period. The zincdithiophosphate produces oil-insoluble precipitates by itsdecomposition. The oil-insoluble precipitates are converted into sludge.When the lubricating oil is a hydraulic oil, the produced sludge causesplugging of filters of the hydraulic system and disturbs normaloperation of the hydraulic system. Moreover, the decomposition of zincdithiophosphate causes lowering of oxidation stability and abrasionresistance of the lubricating oil. Accordingly, the decomposition ofzinc dithiophosphate results in lower durability of the lubricating oil.

[0005] For this reason, it is desired to have a lubricating oilcomposition having high durability, that is, showing high thermalstability and high anti-oxidation property, and producing little sludgewhen it is used under severe conditions, such as at high temperatures,for a long time, and particularly at a high temperature for a long time.

[0006] U.S. Pat. No. 4,263,150 teaches that treatment of mixed metalsalts of dialkyl phosphorodithioic and carboxylic acids with secondaryor tertiary phosphites materially decreases copper corrosion. The ratioof equivalents of phosphorus acid to carboxylic acid may be from about0.1:1 to about 30:1, preferably from about 0.5:1 to about 20:1. Theexamples show a ratio of equivalents of 4:1.

[0007] U.S. Pat. No. 4,308,154 teaches the use of certain metalcarboxylate salts to improve the thermal stability of lubricatingcompositions and functional fluids containing metal dithiophosphatesalts. The ratio of equivalents of dialkylphosphorodithioic acids tocarboxylic acids is between 2.5:1 and 4.5:1.

[0008] U.S. Pat. No. 4,417,990 teaches an additive combinationcomprising metal salts of certain dithiophosphates and carboxylates andsulfurized Group 11 metal phenates. The ratio of equivalents ofdialkylphosphorodithioic acids to carboxylic acids is between 0.5:1 and400:1, but all examples have a ratio of 4:1.

[0009] U.S. Pat. No. 4,466,894 teaches an additive combinationcomprising metal salts of optionally substituted phosphorodithioic acidand C₁-C₂₉ aliphatic or alicyclic carboxylic acid; a sulfurized Group IImetal phenate; and an optionally substituted benzotriazole. The ratio ofequivalents of dialkylphosphorodithioic acids to carboxylic acids isbetween 0.5:1 and 500:1, preferably 0.5:1 and 20:1, but all exampleshave a ratio of 4:1.

[0010] WO Publication No. 84/04322 teaches an additive combinationcomprising metal salts of phosphorodithioic acid and C₂-C₄₀ aliphatic oralicyclic carboxylic acid; a sulfurized Group II metal phenate; and atriazole. The ratio of equivalents of dialkylphosphorodithioic acids tocarboxylic acids is between 0.5:1 and 500:1, preferably 0.5:1 and 20:1,but all examples have a ratio of 4:1.

SUMMARY OF THE INVENTION

[0011] The present invention is based in part on the discovery that theuse of zinc saturated aliphatic monocarboxylate in combination with zincdithiophosphate, when the ratio of equivalents of dithiophosphate tomonocarboxylate is from 6:1 to 45:1, can improve sludge forming tendencyof a lubricating oil composition at high temperature conditions.Preferably, the ratio of equivalents is from 7:1 to 12:1.

[0012] Preferably, the total amount of both zinc salts is in the rangeof 50 to 10,000 ppm expressed in terms of their zinc content, the baseoil is a mineral oil having a viscosity in the range of 2 to 500 cSt at40° C., the zinc dithiophosphate is zinc di-2-ethylhexyldithiophosphate,and the zinc saturated aliphatic monocarboxylate is zinc2-ethylhexanoate.

[0013] In one embodiment, the zinc dithiophosphate and zinc saturatedaliphatic monocarboxylate are in the ratio of 1:0.001 to 1:0.08 byweight, and the total amount of both zinc salts is in the range of 50 to10,000 ppm expressed in terms of their zinc content. Preferably, thezinc dithiophosphate and zinc saturated aliphatic monocarboxylate are inthe ratio of 1:0.008 to 1:0.07 by weight, and the total amount of bothzinc salts is in the range of 100 to 1,000 ppm expressed in terms oftheir zinc content.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to assist the understanding of this invention, referencewill now be made to the appended drawings. The drawings are exemplaryonly, and should not be construed as limiting the invention.

[0015] The FIGURE shows a plot of sludge precipitation period (in hours)for different ratios of equivalents of zinc dithiophosphate to zincsaturated aliphatic monocarboxylate.

DETAILED DESCRIPTION OF THE INVENTION

[0016] In its broadest aspect, the present invention involves alubricating oil composition having a zinc dithiophosphate and a zincsaturated aliphatic monocarboxylate dissolved in a base oil. The ratioof equivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is from 6:1 to 45:1, more preferably from 7:1 to 12:1.

[0017] Preferably, the total amount of both zinc salts is in the rangeof 50 to 10,000 ppm expressed in terms of their zinc content, morepreferably from 100 to 1,000 ppm

The Base Oil

[0018] The lubricating oil composition of the invention is prepared byadding the additives to a base oil in the known manner. The base oil isselected from the known oils for lubricating oils from the viewpoint ofthe purpose of its use, the required performance, and so forth.Therefore, mineral oil is generally used as the base oil. However, ifdesired, synthetic oils such as synthetic hydrocarbon, fatty acidesters, and phosphoric acid ester or vegetable oils such as fatty acidglycerides can be employed. If the lubricating oil composition of theinvention is prepared for the use as a hydraulic oil, the base oilpreferably has a viscosity of 2 to 500 cSt at 40° C.

Zinc Dithiophosphate

[0019] The zinc dithiophosphate can be zinc dialkyldithiophosphate orzinc dialkylaryldithiophosphate. Their derivatives also can be employed.Generally employed zinc dialkyldithiophosphate has an alkyl group of 3to 18 carbon atoms. Preferred alkyl group contains 4 to 12 carbon atoms,and most preferred alkyl group contains 6 to 10 carbon atoms. Generallyemployed zinc dialkylaryidithiophosphate has an alkyl phenyl grouphaving alkyl of 6 to 15 carbon atoms. These zinc dithiophosphates can beemployed singly or in combination. Preferred is the single use of zincdithiophosphate having branched alkyl chain, for instance, zincdi-2-ethylhexyldithiophosphate. Also preferred is a mixture of zincdithiophosphates containing at least 50 wt. % (especially, at least 70wt. %) of the zinc salt of the branched alkyl-type dithiophosphate acid.

[0020] The zinc dithiophosphate is generally produced and supplied inthe form of a solution containing it in mineral oil at a highconcentration. Accordingly, the lubricating oil composition of theinvention is generally prepared using such concentrated solution of zincdithiophosphate. However, the amount of zinc dithiophosphate describedin the specification is calculated on the solid content basis.

Zinc Saturated Aliphatic Monocarboxylate

[0021] The zinc saturated aliphatic monocarboxylate, that is, zinc saltof a couple of a saturated aliphatic monocarboxylic acid, can be zincsalt (normal salt) of a saturated aliphatic monocarboxylic acid having 4to 18 carbon atoms. Preferred is a zinc salt of a couple of saturatedaliphatic monocarboxylic acid having 4 to 12 (particularly 6 to 10). Thezinc carboxylate can be used singly or in combination. Particularlypreferred is the single use of zinc saturated aliphatic monocarboxylatehaving branched alkyl chain, for instance, zinc di-2-ethylhexanoate(herein referred to simply as “zinc di-2-ethylhexanoate”). Alsopreferred is a mixture of zinc saturated aliphatic monocarboxylatecontaining at least 50 wt. % (especially, at least 70 wt. %) of the zincsalt of the branched alkyl-type saturated aliphatic monocarboxylic acid.

[0022] The zinc saturated aliphatic monocarboxylate can be produced andsupplied in the form of a solution containing it in mineral oil at ahigh concentration. Accordingly, the lubricating oil composition of theinvention can be prepared using such concentrated solution of zincsaturated aliphatic monocarboxylate. However, the amount of zincsaturated aliphatic monocarboxylate described in the specification iscalculated on the solid content basis.

Zinc Dithiophosphate and Zinc Saturated Aliphatic MonocarboxylateMixture

[0023] The ratio of equivalents of zinc dithiophosphate to zincsaturated aliphatic monocarboxylate is from 6:1 to 45:1, more preferablyfrom 7:1 to 12:1.

[0024] When the zinc dithiophosphate is zincdi-2-ethylhexyldithiophosphate and the zinc saturated aliphaticmonocarboxylate is zinc 2-ethylhexanoate, the zinc dithiophosphate andzinc saturated aliphatic monocarboxylate are contained in a weight ratioof 1:0.001 to 1:0.08 (former:latter), and the total amount of both zincsalts is in the range of 50 to 10,000 ppm expressed in terms of theirzinc content. If the weight ratio is outside the above range, or if bothsalts are used to give a total zinc content outside the range of 50 to10,000 ppm, it is difficult to attain the object of the invention, thatis, reduction of deterioration of the lubricating oil in the use at hightemperatures and/or for long period of time. Preferred weight ratiobetween zinc dithiophosphate and zinc saturated aliphaticmonocarboxylate is in the range of 1:0.008 to 1:0.07 (former:latter),and preferred total zinc content of both zinc salts is in the range of100 to 1,000 ppm.

Other Additives

[0025] The lubricating oil composition of the invention may furthercontain other additives such as a detergent-dispersant such as a metalsalt of sulfonate, an oxidation inhibitor other than the zincdithiophosphate, a corrosion inhibitor, and an anti-emulsifier. Ifdesired, a pour point depressant such as polymethyl methacrylate can beadded.

[0026] As the additives for lubricating oils, such as thedetergent-dispersants, oxidation inhibitors, rust inhibitors,anti-emulsifiers, and pour point depressants, a variety of additives areknown. These known additives or their analogous compounds can be used,if desired, when the lubricating oil composition of the invention isprepared. Representative examples of the additives are described below.

[0027] A metal phenate or a metal sulfonate is generally used as thedetergent-dispersant. The metal phenate is an alkaline earth metal saltof alkylphenol sulfide having an alkyl group of approximately 8 to 30carbon atoms. Generally employed alkaline earth metals are calcium,magnesium and barium. Preferably the metal sulfonate is an alkalineearth metal salt of a sulfonated aromatic compound or a sulfonatedmineral oil having a molecular weight of approximately 400 to 600.Generally employed alkaline earth metals are also calcium, magnesium andbarium. The metal phenate and metal sulfonate can be used singly or incombination. Also employable are other metal-containing detergents suchas salicylates, phosphorates and naphthenates of alkaline earth metals.These detergent-dispersants can be employed singly or in combination.The aforementioned phenate and sulfonate can be employed in combinationwith these other metal-containing detergents. The metal-containingdetergents can be of a neutral type or of an over-based type having analkalinity value of 150 to 300 or more. If desired, an ashless typedispersant (which may contain boron) can be used singly or incombination.

[0028] The detergent-dispersant is generally used in the lubricating oilcomposition at a concentration of 0.01 to 10 wt. %.

[0029] Examples of oxidation inhibitors other than zinc dithiophosphateinclude various known oxidation inhibitors such as additives ofphenol-type, amine-type, phosphor-containing type and sulfur-containingtype. These oxidation inhibitors can be optionally used.

[0030] The pour point depressant generally is polyalkyl methacrylate.

[0031] The rust inhibitor generally is alkenylsuccinic acid, its salt,ester, or amine derivative.

[0032] The anti-emulsifier generally is polyalkylene glycol.

[0033] The lubricating oil composition of the invention can furthercontain various auxiliary additives other than those mentioned above.Examples of the auxiliary additives include know extreme pressureagents, corrosion inhibitors, friction modifiers, and antifoamingagents. An antiabrasion improver and a multi-functional additive (e.g.,an organic molybdenum compound such as molybdenum dithiophosphate) canbe used in combination.

[0034] The additives can be added simultaneously or separately to a baseoil when the lubricating oil composition of the invention is prepared.Otherwise, the zinc dithiophosphate and zinc saturated aliphaticmonocarboxylate can be added to one portion of a base oil at a highconcentration, and then thus obtained concentrated solution can bediluted with another portion of the base oil to prepare the lubricatingoil composition.

EXAMPLES

[0035] The invention will be further illustrated by following examples,which set forth particularly advantageous method embodiments. While theExamples are provided to illustrate the present invention, they are notintended to limit it.

Example 1

[0036] To 99.596 g of a base oil (mineral oil having a viscosity indexof 98 and showing a viscosity of 32 cSt at 40° C.) were added 0.40 g ofzinc di-2-ethylhexyldithiophosphate and 0.004 g of zinc 2-ethylhexanoateto give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio of 1:0.01 and having a zinc content of 365 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 45.6:1.

Example 2

[0037] The procedure of Example 1 was repeated except that 99.585 g ofthe base oil was used and the amount of zinc 2-ethylhexyldithioate waschanged to 0.015 g to give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.0375 and having a zinc content of 389 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 12.2:1.

Example 3

[0038] The procedure of Example 1 was repeated except that 99.574 g ofthe base oil was used and the amount of zinc 2-ethylhexyldithioate waschanged to 0.026 g to give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.065 and having a zinc content of 389 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 7.0:1.

Example 4

[0039] The procedure of Example 1 was repeated except that 99.57 g ofthe base oil was used and the amount of zinc 2-ethylhexyldithioate waschanged to 0.03 g to give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.075 and having a zinc content of 422 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 6.1:1.

Comparison Example A

[0040] The procedure of Example 1 was repeated except that 99.60 g ofthe base oil was used and zinc 2-ethylhexanoate was not used to give alubricating oil composition containing di-2-ethylhexyldithiophosphateonly and having a zinc content of 356 ppm.

Comparison Example B

[0041] The procedure of Example 1 was repeated except that 99.559 g ofthe base oil was used and the amount of zinc 2-ethylhexanoate waschanged to 0.041 g to give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.103 and having a zinc content of 446 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 4.5:1.

Comparison Example C

[0042] The procedure of Example 1 was repeated except that 99.555 g ofthe base oil was used and the amount of zinc 2-ethylhexanoate waschanged to 0.045 g to give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.113 and having a zinc content of 445 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 4.05:1.

Comparison Example D

[0043] The procedure of Example 1 was repeated except that 99.435 g ofthe base oil was used and the amount of zinc 2-ethylhexanoate waschanged to 0.165 g to give a lubricating oil composition containingdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.41 and having a zinc content of 719 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 1.11:1.

Thermal Stability Test

[0044] (1) Test Method

[0045] In a 50 ml-beaker was placed 40 ml of a lubricating oilcomposition sample. The beaker was then placed in a forced-aircirculation oven controlled at 160° C.±1° C. The oil composition samplewas taken out every one hour to check precipitation of sludge anddetermine the time when sludge began to precipitate. This test meansthat thermal stability of a lubricating oil composition is higher in thecase that the period is longer.

[0046] (2) Test Results Examples Com. Examples 1 2 3 4 A B C DEquivalent 45.6 12.2 7.0 6.1 — 4.5 4.05 1.11 Ratio Sludge 88 96 90 86 4656 55 48 Precipitation Period (hours)

[0047] The above results indicate that the lubricating oil compositionsof the invention containing zinc dithiophosphate and zinc carboxylate inthe specific ratio shows thermal stability higher than the lubricatingoil composition containing only zinc dithiophosphate or containing thezinc carboxylate at a large amount.

[0048] Note that the sludge formation time is at least 53% better whenthe ratio of equivalents of zinc dithiophosphate to zinc saturatedaliphatic monocarboxylate is from 6:1 to 45:1, and that best results areobtained with ratios of from 7:1 to 12:1.

Example 5

[0049] To 99.216 g of a base oil (mineral oil having a viscosity indexof 98 showing a viscosity of 32 cSt at 40° C.) were added 0.40 g of zincdi-2-ethylhexyldithiophosphate and 0.004 g of zinc 2-ethylhexanoate andfurther 0.10 g of 2, 6-di-tert-p-cresol, 0.20 g of basic calciumpetroleum-sulfonate, 0.07 g of tetrapropenylsuccinic acid, and 0.01 g ofpolyoxyalkylene anti-emulsifier, to give a lubricating oil compositioncontaining di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at aweight ratio of 1:0.01 and having a zinc content of 365 ppm. The ratioof equivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 45.6:1.

Example 6

[0050] The procedure of Example 5 was repeated except that 99.205 g ofthe base oil was used and the amount of zinc 2-ethylhexanoate waschanged to 0.015 g to give a lubricating oil composition containing zincdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.0375 and having a zinc content of 389 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 12.

Example 7

[0051] The procedure of Example 5 was repeated except that 99.194 g ofthe base oil was used and the amount of zinc 2-ethylhexyldithioate waschanged to 0.026 g to give a lubricating oil composition containing zincdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.065 and having a zinc content of 413 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 7.0:1.

Example 8

[0052] The procedure of Example 5 was repeated except that 99.19 g ofthe base oil was used and the amount of zinc 2-ethylhexyldithioate waschanged to 0.03 g to give a lubricating oil composition containing zincdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.075 and having a zinc content of 422 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 6.1:1.

Comparison Example E

[0053] The procedure of Example 5 was repeated except that 99.22 g ofthe base oil was used and zinc 2-ethylhexanoate was not used to give alubricating oil having a zinc content of 413 ppm.

Comparison Example F

[0054] The procedure of Example 5 was repeated except that 99.179 g ofthe base oil was used and the amount of zinc 2-ethylhexanoate waschanged to 0.165 g to give a lubricating oil composition containing zincdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.103 and having a zinc content of 719 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 4.5:1.

Comparison Example G

[0055] The procedure of Example 5 was repeated except that 99.055 g ofthe base oil was used and the amount of zinc 2-ethylhexanoate waschanged to 0.165 g to give a lubricating oil composition containing zincdi-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weightratio 1:0.41 and having a zinc content of 719 ppm. The ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is about 1.11:1.

Evaluation Test as Hydraulic Oil

[0056] (1) Thermal Stability Test ASTM-D-2070 (Cincinnati MillacronThermal Stability A Method) In this test, less production of sludge (mgof sludge production per 100 ml of test oil) means a lubricating oil ofhigher thermal stability.

[0057] (2) Oxidation Stability Test

[0058] JIS-K-2514 (according to this rotary cylinder type oxidationstability test method:150° C.)

[0059] In this test, a longer life time before oxidation means ahydraulic oil having higher oxidation stability.

[0060] (3) Abortion Resistance Test

[0061] JIS-K-2514 (according to the oxidation stability test forinternal engine lubricating oil: test oil was deteriorated by keeping itat 150° C. for 24 hours, and the deteriorated oil was placed in a shellfour ball type friction abrasion test machine and subjected tofriction-abrasion at a pressure of 30 kgf, a revolution of 1,800 r.p.m.for 30 min.)

[0062] In this test, a smaller wear track diameter means a lubricatingoil having a higher friction resistance.

[0063] (4) Test Results Examples Com. Examples 5 6 7 8 E F G EquivalentRatio 45.6 12.2 7.0 6.1 — 4.5 1.11 Thermal Stability Test 3 1 2 5 153 2395 Amount of Sludge (mg) Oxidation Stability 355 341 343 340 352 321 265Test Life Time (min.) Abrasion Resistance Test Wear Track Diameter (mm)0.52 0.56 0.60 0.60 1.20 0.64 0.66

[0064] The above results indicate that the lubricating oil compositionsof the invention containing zinc dithiophosphate and zinc carboxylate inthe specific ratio shows better balanced characteristics in thermalstability, oxidation stability and abrasion resistance, than thelubricating oil composition containing only zinc dithiophosphate orcontain the zinc carboxylate at a large amount.

[0065] The lubricating oil composition of the invention is high in hightemperature stability and oxidation stability and produces less sludgein its use at high temperatures or for a long period of time,particularly under sever conditions such as a combination of hightemperatures and prolonged use. Accordingly, the lubricating oilcomposition of the invention has excellent durability, and isadvantageously employable for lubricating hydraulic systems and powertransmission devices.

[0066] While the present invention has been described with reference tospecific embodiments, this application is intended to cover thosevarious changes and substitutions that may be made by those skilled inthe art without departing from the spirit and scope of the appendedclaims.

What is claimed is:
 1. A lubricating oil composition comprising a zincdithiophosphate and a zinc saturated aliphatic monocarboxylate dissolvedin a base oil, wherein the ratio of equivalents of zinc dithiophosphateto zinc saturated aliphatic monocarboxylate is from 6:1 to 45:1.
 2. Alubricating oil composition according to claim 1 wherein the ratio ofequivalents of zinc dithiophosphate to zinc saturated aliphaticmonocarboxylate is from 7:1 to 12:1.
 3. A lubricating oil compositionaccording to claim 1 wherein the total amount of both zinc salts is inthe range of 50 to 10,000 ppm expressed in terms of their zinc content.4. The lubricating oil composition of claim 1 , wherein the base oil isa mineral oil having a viscosity in the range of 2 to 500 cSt at 40° C.5. The lubricating oil composition of claim 1 , wherein the zincdithiophosphate is zinc di-2-ethylhexyldithiophosphate.
 6. Thelubricating oil composition of claim 1 , wherein the zinc saturatedaliphatic monocarboxylate is zinc 2-ethylhexanoate.
 7. A lubricating oilcomposition comprising a zinc dithiophosphate and a zinc saturatedaliphatic monocarboxylate dissolved in a base oil, wherein the zincdithiophosphate and zinc saturated aliphatic monocarboxylate are in theratio of 1:0.001 to 1:0.08 by weight, and the total amount of both zincsalts is in the range of 50 to 10,000 ppm expressed in terms of theirzinc content.
 8. The lubricating oil composition of claim 7 , whereinthe zinc dithiophosphate and zinc saturated aliphatic monocarboxylateare in the ratio of 1:0.008 to 1:0.07 by weight, and the total amount ofboth zinc salts is in the range of 100 to 1,000 ppm expressed in termsof their zinc content.